Abstract: A process and apparatus for indirectly heating an endothermic reaction by combustion of reactants or products from the endothermic reaction using a plate heat exchange arrangement in a highly efficient manner. This invention is particularly suited for processes such as the production of styrene or synthesis gas. When producing synthesis gas, oxidizing reactants in a secondary reforming step generates heat for a primary reforming step and the process improves selectivity and yield with a highly efficient heat exchange step that uses narrow channel for indirect heat exchange. The narrow channels are preferably defined by corrugated plates. The primary reaction channels will contain a catalyst for the promotion of the primary reaction such as steam reforming or ethylbenzene dehydrogenation.